Patent GB-683951-A (GB)
Title: Improvements in electric motor control systems
IPC: G05D3/18
Filed: 1949-07-28
Abstract: 683,951. Control of A.C. motors. BRITISH THOMSON-HOUSTON CO., Ltd., LUDBROOK, L. C., and MAGGS, A. H. July 28, 1949, No. 19877/49. Class 38 (iii) [Also in Group XXXV] Relates to the reversing systems for A.C. motors wherein the appropriate energisation of the motor windings is effected by means of saturable reactors connected between the motor windings and the A.C. source, the impedance of the reactors being controllable by a D.C. signal of selected polarity. This signal is modified by combining with it a feed-back direct voltage which is derived from the motor windings and has a magnitude and polarity such as progressively to decrease the control signal. As shown, a single-phase commutator motor has an armature A, having short-circuited brushes, and stator windings S1, S2. The winding S2 is supplied through a bridge arrangement of saturable reactors of which windings X1, X2, are placed on the same core and are controlled by winding X3, windings Y1, Y2, being similarly controlled by winding Y3. When the bridge is balanced no excitation is applied to winding S2. If, however, a direct control voltage is applied to winding X3 or Y3 the bridge is unbalanced and the winding S2 is energised. The windings X3, Y3, have a common control circuit, rectifiers R being so poled that a control signal of chosen polarity energises the appropriate winding for motor rotation in a selected direction. The linear relation between the strength of the control signal and the torque of the motor is upset by changes of impedance of the bridge network with variations of the signal, and, in order to improve linearity, a voltage derived from winding S2 is fed back to the direct-current control circuit. A demodulator DM of the type described in Specification 663,277, [Group XXXVIII], is employed and has input voltages derived from windings S1, S2, and a direct output voltage having polarity and magnitude dependent upon the phase relation of the two input voltages. The feedback voltage, which reduces the signal voltage at its higher values, may be modified by a component dependent upon the current in winding S2, derived through current transformer CT. To prevent undesired heating of the winding S2 at low speeds the reactors in the bridge circuit may be biased so that initially the bridge has low impedance. One way in which this may be effected is described in Specification 684,020. In an alternative arrangement the phase of the excitation of the winding S1 is automatically maintained in quadrature with that applied to winding S2 thus enabling substantially the whole of the current in the winding S2 to produce useful torque. To this end, a saturable reactor, which is connected in a phase-shift circuit through which winding S1 is supplied, has its control winding energised by the direct-current control signal, Fig. 2 (not shown). In a modification, Fig. 3 (not shown), the components of flux in time-phase and space quadrature are produced by interaction between an alternating-current excited stator field winding and an armature winding on the rotor which has two pairs of brushes displaced on either side of the axis of the field winding. Each pair of brushes is inter-connected through reactors the degree of saturation of which is controlled by a direct potential of reversible polarity, whereby progressive shortcircuiting of one or other of the pairs of brushes is effected for reversible operation of the motor. A feed-back signal is applied to the control windings of the reactors and comprises components dependent upon the armature current and speed of the motor, the former being derived through a current transformer and the latter from a motor-driven tachometer generator. In Fig. 4 (not shown) a motor having a pair of brushes arranged co-axially with a stator winding, and additional opposing field windings, is employed. Specification 390,165, [Group XXXV], also is referred to.